This invention relate to diffractive devices and to their manufacture and has particular though not exclusive application to the provision of optically variable security diffractive devices which may be adapted for affixment to or incorporation in, for example, currency notes, credit cards, charge cards, share certificates and the like.
The present applicant""s international patent publication WO91/03747 (application PCT/AU90/00395) proposes a diffraction grating structure comprised of a multiplicity of pixels which are individual optical diffraction gratings so that the pixellated diffraction grating when illuminated generates an optically variable image. The applicant""s pixellated diffraction gratings utilising curved line grating pixels have become known by the trademark Pixelgram (trade mark). According to preferred aspects of the arrangement disclosed in the international application, the respective diffraction grating of each grating pixel comprises a plurality of reflective or transmissive grooves or lines which are usually curved across a pixel. Groove or line curvature determines both local image intensity, e.g. shading, and local optical structural stability. Groove or line spacing in each pixel determines local colour properties, with non-primary colours generated by a pixel mixing. Average groove or line orientation determines movement or colour effects, and the number of distinct values of average curvature and average spacing may be viewed as defining the Pixelgram palette, by analogy with the language of computer graphics. A further disclosure of a security diffraction grating structure is to be found in international patent publication WO90/07133 (PCT/AU89/00542).
The present applicant""s international patent publication WO93/18419 (PCT/AU93/00102) discloses how selected visually observable effects in the optically variable image may be generated by arraying the pixels in groups within which the pixels are arranged according to a predetermined rule for the pixellated diffraction grating. Thus, for example, multiple sets of different images, or of the same image but different shading or colour, may be produced at different viewing angles.
The concept of providing multiple optically variable images at different viewing angles, using a pixellated diffractive device in which each pixel contains a sub-pixel corresponding to each image, is also disclosed in U.S. Pat. No. 5,032,003. In that case, each diffractive sub-pixel is a straight line grating. This is an example of a more general class of pixellated diffractive structures utilising straight line grating pixels and known by the trademark Kinegram.
Australian patent application 10499/92 proposes a pixellated diffraction grating structure with three channels which constitute views from different angles of the same image, in order to obtain a stereoscopic image. The gratings may be curved line gratings. A predecessor of this reference is Japanese patent (Kokai) publication 2-72320.
European patent publication 467601 is concerned with holographic diffraction grating patterns which may include curved line gratings. Overlaid or alternate channels are proposed for providing different images at different angles. The different images may include numerical information and logos.
The present inventor has now appreciated that the concepts of the aforementioned applications can be further extended to provide diffractive devices which give one or more optically variable images, by fracturing the pixels of each image into sub-pixels and then rearranging and interlacing the sub-pixels so that the sub-pixels cooperatively provide elements of the respective images. In proposing this further development, the inventor has appreciated that he can take advantage of the mathematical theorem in Fourier analysis that the Fourier transform of any diffractive function is translationally invariant.
The invention accordingly provides, in one aspect, a pixellated diffractive device comprising a multiplicity of pixels in turn divided into multiple sub-pixels, which device is related to one or more pixellated diffraction surface structures which when illuminated generate respective corresponding optically variable images. Sub-pixels of each pixel of the diffractive device include or consist of diffractive elements arranged in one or more groups. The diffractive elements of each group match diffractive elements of a corresponding single pixel of the respective pixellated diffraction surface structures. In each pixel of the device the diffractive elements of the or each said group are intermixed with other sub-pixels and cooperatively contribute a single element of the corresponding optically variable image which is generated on illumination of the diffractive device.
The invention also provides, in another aspect, a method of deriving at least a representation of a pixellated diffractive device, comprising a multiplicity of pixels in turn divided into multiple sub-pixels, which method comprises deriving at least a primary representation of each of one or more pixellated diffraction surface structures which when illuminated generate respective corresponding optically variable images, fracturing each pixel of the or each said primary representation into multiple diffractive elements, and deriving at least a secondary representation of said pixellated diffractive device by forming each pixel thereof so that sub-pixels thereof include or consist of diffractive elements arranged in one or more groups, the diffractive elements of each group matching diffractive elements of a corresponding single pixel of the respective said pixelated diffraction surface structure, wherein in each pixel of the device, the diffractive elements of the or each said group are intermixed with other sub-pixels and cooperatively contribute a single element of the said corresponding optically variable image.
In some prior references, the term xe2x80x9crelief structurexe2x80x9d is utilised interchangeably with or instead of xe2x80x9cdiffraction gratingxe2x80x9d or xe2x80x9cdiffraction surface structurexe2x80x9d. The term xe2x80x9cdiffraction surface structurexe2x80x9d is employed herein to indicate a structure which is either reflective or transmissive. Without in any way limiting the scope of xe2x80x9cdiffraction surface structuresxe2x80x9d, it is noted that such structures may include, for ample, line or groove diffraction gratings, small squares, rectangles or polygons.
By xe2x80x9cat least a representationxe2x80x9d is meant that the respective integer may be actually formed, or, if not, at least a representation is formed. The representation may be a set of code or data defining the respective integer, e.g. in a computer memory means. The aforesaid deriving steps are preferably carried out in suitably programmed computer operations. The method may advantageously include the step of utilising the derived representation to drive a suitable machine, e.g. an electron beam lithography machine, to form the actual diffractive device.
Preferably, there are at least two optically variable images, each associated with a respective group of diffractive elements. The images may be the same or similar scenes but differently oriented or of different shading or colour. One or more further groups of sub-pixels of the device may collectively generate an optically invariable image.
There are preferably at least four sub-pixels per pixel, but more preferably at least 16 in a 4xc3x974 square array of square sub-pixels. The pixels are preferably sufficiently small to be below the resolution limit of a healthy human eye, for example, less than 125 micron on edge and more preferably about 30 to 80, e.g. around 60 micron.
The diffractive sub-elements are preferably dispersed within each pixel so as to produce a predetermined discernible effect in the corresponding optically variable image. The dispersal of the diffractive elements may be chosen from a predetermined set of selections which therefore defines a mapping palette for the diffractive element array, again by analogy with the language of computer graphics. In a case where, in accordance with international patent publication WO91/03747, the respective diffraction surface structures forming the pixels of the pixellated diffraction surface structure have been formed, e.g. in relation to predetermined variables such as groove or line curvature, groove or line spacing and average groove or line orientation, from a primary palette, the aforementioned mapping palette forms a secondary palette and the diffractive device entails successive selections from both the primary and secondary palettes.
The diffractive elements within the pixels may involve classes of miniature diffraction gratings of curved and variably spaced grooves, or alternatively elementary arrays of polygon shaped relief structures (pixels within pixels of pixels or SQUOTS) of dimensions of the order of fractions of a micron. In general the transformed representations of the input images may involve classes of groups of diffractive elements. A class of different groups of diffractive elements is defined as a component palette of component pixel types. Since each element of a component palette is itself a group of diffractive elements, each diffractive element can be regarded as a member of a sub palette and hence each component palette is, in this embodiment, a palette of sub palettes (palettes within palettes).
The invention therefore also provides, in a further aspect, a method of forming a pixellated diffractive device in terms of a multiplicity of diffraction grating palettes wherein each palette contains miniature diffraction grating groups and each group M is in itself a sub-pixel palette of N sub-pixel diffractive elements comprising defining the device by repeating each group M at predetermined locations within a large array of repeat group locations, the map of repeat group locations for each group M being determined by a set of complex mapping relations between the large array and a set of invariable image component maps which act co-operatively under the control of the mapping relations to define the diffractive properties of the diffractive device thereby formed on said large array.
As already indicated the images generated by the diffractive device may be either optically invariable or optically variable. An image is described herein as xe2x80x9coptically variablexe2x80x9d where it varies according to the position of observation and xe2x80x9coptically invariablexe2x80x9d where it remains substantially the same regardless of the position of observation. By xe2x80x9cimagexe2x80x9d in the context of this specification is meant the optical image observed by the naked eye focussed on the diffractive device when it is illuminated by an arbitrarily extended diffuse source of finite width such as a fluorescent tube. The term xe2x80x9cimagexe2x80x9d is used herein in its broadest sense, not being limited to pictorial or diagrammatic images but extending, e.g. to figures, numbers, data and codes.
It is an advantage of the present invention that the diffractive device may produce not only a mix of optically variable and optically invariable images but may also produce two or more different kinds of optically variable images. For example, the optical variable images may be of a Pixelgram type (i.e. each pixel is a cure line diffraction grating), a Kinegram type (i.e. each pixel is a straight line diffraction grating), or a mathematical hologram. For each case, the structure of the component mapping palettes is determined by the optical properties of the input images and the required optical properties of the component viewable images generated by the diffractive device.
The diffraction device may be provided on a suitable substrate, eg a metal foil, and/or may be affixed or formed in a carrier eg a currency note, credit card, bank account or ATM card, debit card, security card, charge card or prepaid card.